fix(email): test saved OAuth accounts through shared transports

[RaresKeY]

Summary

Let the saved-account Test connection action authenticate Google OAuth
accounts without requiring stored IMAP/SMTP passwords. The route resolves
OAuth only from the owner-checked saved row, keeps inline form values on the
password path, and reports the real IMAP and SMTP results independently.

This replacement preserves TNTBA's two focused commits from #5021 and keeps
the current-dev owner and authentication updates in a separate maintainer commit.

Related implementation

Supersedes #5021 on current dev while preserving its original authored commits.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #5644

Part of #5637

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and open PRs — fix(email): use XOAUTH2 in test-connection endpoint for Google OAuth accounts #5021 is the stale focused implementation this branch preserves and replaces.
• This PR targets dev.
• My changes are limited to saved-account connection testing, OAuth-aware authentication, and focused regressions.
• I ran the app end-to-end. No disposable Google/IMAP/SMTP account was available; protocol behavior was validated in isolated tests with mocked provider/network I/O.

How to Test

1. Run:

   python -m pytest -q \
     tests/test_email_test_connection_oauth.py \
     tests/test_email_oauth.py \
     tests/test_email_imap_timeout.py \
     tests/test_email_smtp_security.py \
     tests/test_email_polly_imap_leak.py \
     tests/test_imap_leak_fixes.py

2. Save a Google OAuth account with no IMAP/SMTP password and run Test connection.

3. Verify IMAP and SMTP each use the saved account identity and report success independently.

4. Repeat with a password account and verify its login behavior is unchanged.

Fresh current-dev review passes 137 broader email/IMAP/SMTP tests and Python compilation. Diff checks pass. Live-provider validation remains a reviewer item.

Visual / UI changes

N/A — backend connection testing and tests only; no UI rendering changed.

Screenshots / clips

N/A — no visual changes.